I’m trying to figure out what might cause Cowboy to consume a lot of memory when sending JSON data (about 3.5 megs) as iodata through Plug, but with JSON string the memory footprint is very small.

I have a schema with around 10 fields and three of “many” type of associations as a list of around 12.2k items. I noticed that when I do render(conn, "index.json", fairly_large_list: list) in the controller and make a request, Observer shows that cowboy_clear:connection_process/4 consumes almost 50 megs of memory when running cowboy_http:loop/1.

So I started to dig a bit deeper and replaced the standard render with the following

json =
  Phoenix.View.render_to_iodata(MyView, "index.json", %{fairly_large_list: list})

conn
  |> Plug.Conn.put_resp_header("content-type", "application/json")
  |> Plug.Conn.send_resp(200, json)

Running the request again shows the same memory consumption. However, when I change that to

json =
  Phoenix.View.render(MyView, "index.json", %{fairly_large_list: list})
  |> Jason.encode!()
  
conn
  |> Plug.Conn.put_resp_header("content-type", "application/json")
  |> Plug.Conn.send_resp(conn.status || 200, json)

Then the Cowboy process maxes to around 40k of memory.

So basically, when I send an iolist, Cowboy consumes 1000 times more memory with iolist than if I send a string. I took response compression off, but it didn’t have any effect. The frontend is directly connected to Cowboy.

Any ideas? Thanks!